Page 29 - Modern Analytical Chemistry

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12 Modern Analytical Chemistry

2A Numbers in Analytical Chemistry

Analytical chemistry is inherently a quantitative science. Whether determining the
concentration of a species in a solution, evaluating an equilibrium constant, mea-
suring a reaction rate, or drawing a correlation between a compound’s structure
and its reactivity, analytical chemists make measurements and perform calculations.
In this section we briefly review several important topics involving the use of num-
bers in analytical chemistry.

2A.1 Fundamental Units of Measure
Imagine that you find the following instructions in a laboratory procedure: “Trans-
fer 1.5 of your sample to a 100 volumetric flask, and dilute to volume.” How do you
do this? Clearly these instructions are incomplete since the units of measurement
are not stated. Compare this with a complete instruction: “Transfer 1.5 g of your
sample to a 100-mL volumetric flask, and dilute to volume.” This is an instruction
that you can easily follow.
Measurements usually consist of a unit and a number expressing the quantity
of that unit. Unfortunately, many different units may be used to express the same
physical measurement. For example, the mass of a sample weighing 1.5 g also may
be expressed as 0.0033 lb or 0.053 oz. For consistency, and to avoid confusion, sci-
entists use a common set of fundamental units, several of which are listed in Table
SI units 2.1. These units are called SI units after the Système International d’Unités. Other
Stands for Système International d’Unités. measurements are defined using these fundamental SI units. For example, we mea-
These are the internationally agreed on sure the quantity of heat produced during a chemical reaction in joules, (J), where
units for measurements.
2
mkg
1 J = 1
s 2
Table 2.2 provides a list of other important derived SI units, as well as a few com-
monly used non-SI units.
Chemists frequently work with measurements that are very large or very small.
A mole, for example, contains 602,213,670,000,000,000,000,000 particles, and some
analytical techniques can detect as little as 0.000000000000001 g of a compound.
scientific notation For simplicity, we express these measurements using scientific notation; thus, a
A shorthand method for expressing very mole contains 6.0221367 ´10 particles, and the stated mass is 1 ´10 –15 g. Some-
23
large or very small numbers by
indicating powers of ten; for example, times it is preferable to express measurements without the exponential term, replac-
–15
3
1000 is 1 ´10 . ing it with a prefix. A mass of 1 ´10 g is the same as 1 femtogram. Table 2.3 lists
other common prefixes.

Table 2.1 Fundamental SI Units
Measurement Unit Symbol

mass kilogram kg
volume liter L
distance meter m
temperature kelvin K
time second s
current ampere A
amount of substance mole mol